Electronics for a thin bed array induction logging system

ABSTRACT

A logging tool electronics system is disclosed (FIG.  1 ) with noise minimization features and pulse compression signal processing techniques to improve the signal-to-noise ratio of array induction logging tools. The borehole is radiated with a magnetic field produced by a configurable multi-frequency sine wave signal stimulus section driving a fully differential single transmitter coil. Received signals from multiple mutually balanced fully differential receiver arrays are processed by receiver signal chains using adaptive algorithms under firmware control. The received signals are used to determine the conductivity and resistivity of the formation surrounding the borehole.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to the field of logging of oil, gas and mineralwells. More particularly, improvements relating to real-time signalprocessing and pulse compression for multi-frequency array inductionlogging are disclosed.

2. Description of Related Art

Induction logging tools are instruments used in logging operations inboreholes that are drilled into underground rock formations in thesearch for oil, gas or minerals. These tools measure the electricalconductivity of rock formations to determine the presence and the amountof desired minerals in a potential pay zone. Oil and natural gas causethe rock to have a lower than usual conductivity because these fluidsare electrically non-conducting and they displace connate saline water.Induction logging tools ideally provide accurate quantitative measuresof the fractional saturation of oil or gas in the pay-zone.

Induction logging tools employ sensor arrays that map the rockconductivity at various radial distances from the borehole, so that theinfluence of invasion of borehole fluids may be reduced. These toolsoperate on the principle of induced eddy currents, which aresubstantially proportional to conductivity, and which may be excited anddetected using sensitive coils. Tools that are known in the art usearrays of coils that provide capabilities to sense conductivity todifferent radial distances from a wellbore.

U.S. Pat. No. 5,157,605 discloses an induction logging method andapparatus for operating an induction sonde at a plurality of frequenciessimultaneously. A plurality of two-coil receiver arrays are used. U.S.Pat. No. 5,548,219 discloses a system for generating multiplefrequencies for heterodyne measurement system for use in logging. U.S.Pat. No. 7,183,771 discloses a device comprising a circuit for injectinga calibration signal into the receivers to obtain measurements free fromerrors introduced by the receiving elements of the system.

Despite many advances in induction logging tool technology, severalelectronics-related problems remain to be solved. For example,electronics signal fidelity issues are still observed in the noisyenvironment of downhole logging, especially when highly conductiveformations are present. Further problems for induction logging tools arecaused by the “skin effect.” Skin effect causes a loss ofproportionality between a received signal and formation conductivity,thereby making interpretation of signals from induction logging toolsmore complex. Conversely, very low conductivity rocks present accuracyproblems for induction logging tools due to low signal-to-noise ratios.These problems are made more challenging when the beds or rockformations of interest are relatively thin.

Attempts to resolve these problems have exhibited shortcomings,including a high cost/benefit ratio. What is needed is a cost-effective,robust, electronics subsystem with a design centered on noiseminimization/cancellation resulting in improvements in data signalfidelity and more accurate logging results.

BRIEF SUMMARY OF THE INVENTION

Pulse compression signal processing techniques and fast, high-resolutionmulti-measurements are used to improve the signal-to-noise ratio bymodulating the waveform driving the transmitter and correlating thereceived signal with the transmitted signal. For the source ofradiation, a digital highly-phase-stable, low-distortion sine wavegenerator/power amplifier for energizing a differentially driven singletransmitter coil, parallel-tuned to desired frequencies at selectedamplitudes and repetition rates, is disclosed. The transmitter coilinduces current flow into the formations around the wellbore.

To measure induced current from the formation, several mutually balancedfully differential receiver coils, located at different distances fromthe transmitter, are coupled to a wideband, low-noise,receiver-amplifier signal chain using a preamplifier/amplifier section,followed by signal processing blocks that include a selective bandpassfilter/phase sensitive detector section, and a high-sampling-rate 24-bitADC converter controlled by a high-speed microcontroller. All receiversare accessed, calibrated and synchronized in parallel by a real-timeprocessor that collects data, drives the tool and communicates with thetelemetry system.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 shows a general system architecture block diagram of the downholeelectronic section.

FIG. 2 shows a typical multi-frequency pulse width and pulse repetitionperiod.

FIG. 3 illustrates the phase differences of the electrical currents,magnetic field and voltage.

FIG. 4 shows the system architecture with optional features.

FIG. 5 shows the system control firmware flowchart.

DETAILED DESCRIPTION OF THE INVENTION

The logging tool electronics disclosed herein is for an array inductiontool composed of five receivers and one transmitter coil, but could beused for different topologies with a different number oftransmitter/receiver coils. The receivers are preferably used in pairs,with one coil “bucking” the other (the coils are wound in oppositedirections). The formation is radiated with the generated magnetic fieldproduced by a highly-configurable digital waveform generator/transmitterdriver/power amplifier section (the transmitter signal stimulussection), as illustrated in FIG. 1, with pulse compression signals, asillustrated in FIG. 2. Waveform generator 10 and amplifier 11 arepreferably a digitally controlled, highly-phase stable, low distortionsine wave generator and power amplifier. They energize parallel-tuneddifferentially driven transmitter coil 13 at a single frequency or in afrequency sweep mode (chirp) for multi-frequency operation. The phaseaccuracy of generator 10 and the temperature stability of master clock10 a are designed for stability over a broad range of wellboretemperature. The apparatus generates a steady-state sinusoidal pulse ofamplitude (A) at each frequency. The amplitudes (A) at each frequencymay be independently controlled. The carrier frequencies (16 KHz, 24 KHzand 32 K Hz, for example) are truncated by the pulse width, or a pulsecompressed magnetic field. The transmitter current is sensed by coil 14and read by voltage across resistor 14 a, which serves as an internalreference.

The pulse width, as illustrated FIG. 2, may be 50 milliseconds. Theenveloped sinusoidal pulses at three frequencies (pulse compression)achieve different depths of investigation. Eddy currents generated inthe formation are detected using receiver coils 16 (FIG. 1) located atdifferent distances from the transmitter coil. A highly-configurablereceiver signal chain, consisting of receiver coils 16, amplifier 17,band-pass filter 18 and amplifier 19, processes in-phase (zero phasedifference with transmitter current) and quadrature (90 degrees out ofphase with transmitter current) components, as shown at 20 and 21. Thesignal is then digitized, preferably using 24-bit A/D conversiontechnology (signal measurement section), as illustrated at 22. Themeasured eddy currents in the formation indicate the formationresistivity. Using adaptive algorithms under firmware control,reconfigurable stimulus and measurement components are controlled.Adaptive algorithms can be stored in a computer-readable medium indownhole hardware in the housing of a logging tool or in surfacehardware. The downhole system disclosed herein is well suited for use ofadaptive algorithms to set stimulus and measurement components in accordwith optimum operating conditions of the system as determined by thealgorithms. Adaptive algorithms for optimization are well known.Frequency is selected and control of ground and logging configurationare achieved at 23, under control of master control 24. A data bus fromtelemetry is connected to the input of master control 24 and signalsfrom the master control are put on an internal data bus to receivers.

FIG. 3 illustrates the phase angles of the signals and responses, where:

B_(T)=transmitter magnetic field,

I_(T)=transmitter current,

I_(L)=formation current “ground loop,”

B_(L)=formation current “ground loop” magnetic field,

R=in-phase receiver DAQ (Digital Acquisition) voltage component due toformation conductivity, and

X=quadrature phase receiver voltage component due to formation skineffect.

The formation “ground loop” current flows around the induction tool dueto coupling with the magnetic field generated by the transmitter.Formation conductivity is determined by the formation ground loopcurrent, which generates a secondary magnetic field, that couples asignal into the receiver array that is an indicator of formationconductivity (or inversely, resistivity). Current flow in thetransmitter coil establishes the primary (reference) magnetic fieldgenerated by the transmitter.

The ratio of the sensed X-signal with respect to the R-signal at thereceiver coil for a high performance auto-shielded induction tool can bearound 10:1 in conductive formations and boreholes, so digitizedR-signal (in phase) formation data is the signal of interest inlow-conductivity formations, and quadrature X is used for calibration,skin effect correction and in special processing algorithms inhigh-conductivity environments.

The transmitter signal stimulus section generates the magnetic field,driving controlled pulsing current at transmitter coil 13. Thetransmitter and selected capacitors create tuned tank 12, whichfunctions according to the equation:f=½π√LCwhere f is the operation frequency, L the transmitter inductance, and Cis the capacitance of a variable capacitor controlled by firmware. Theelectronics, switching frequency and capacitors are preferably selectedevery 10 mS, and the pulse frequency of operation is adjustable byfirmware from 1K to 32 KHz.

The receiver coil measurement signal chain DAQ system, (16, 17, 18, 19,20, and 21) shown in FIG. 1 measures the auto-induced signals andapparent conductivity using a phase-sensitive detector based on squarewave reference signals auto-generated in the stimulus section. The DAQreconfigures the bandpass filters per frequency, grounds the receiverfront end and measures ground and a reference channel periodically tocalibrate downhole, as will be explained below.

All R and X signals from five receivers and calibration signals arecollected in parallel in real time and may be used to correlate andcalculate the real conductivity point-by-point up-hole (on the surface).Master processor 24 drives the tool operation and collection of datausing a widely-used industrial network bus, discussed below.

The tool is designed to operate over the industrial network bus,transmit data up to 1M bit and support up to 32 nodes. This structureprovides the flexibility to manage different kinds of induction tools,with multiple transmitters and multiple receivers, and be adjustable infrequency or capable of sweeping frequencies. In the design discussedhere, the tool includes five receivers (five pairs of receivers, main(+) and bucking (−) in series at each spacing), one transmitter and ithas three operating frequencies. This tool also supports the connectionof several kinds of sensors and actuators, only needing an address andfirmware to be accessed by the master processor. Each module connectedto the data buss (MUX, RECEIVERS, CONTROL and TRANSMITTER) hasreconfigurable, auto-calibration, and auto-test features that make foradjustable and adaptable electronics for several kinds of tools,topologies, and configurations.

This multicore architecture was developed to minimize system stabilityissues, allowing every module enough autonomy to improve signalmeasurement dynamic range (for low conductivity formation byover-sampling the point in order to reduce the noise and get betterdata) and scale power consumption with sampling frequency, which isparticularly important in newer tool designs using lower power supplyvoltage components. Recent developments in adaptive (reconfigurable)systems and the use of higher sampling rate ADC's are combined toprovide this robust high-speed architecture not available in previoussystems.

These features enable electronics with the capability to avoid high andlong transients in the stimulus section, permit sweeping in multiplefrequencies (while keeping the same depth of investigation of previoussystems) and the capability to use several receivers. Faster samplingallows the system to have higher resolution in thin bed high-definitionlogging.

Generically, array induction tool measurement systems are performancelimited due to the difficulty in configurability to a wide variety ofanalog circuit requirements. A highly-programmable analog system thatcan be configured for arbitrary analog functionality is quite valuable.This includes the tool's ability to sweep in frequency, switchingcapacitors at the transmitter, selecting frequency of operation at thereceivers, reconfiguring resistors to adjust gain, and changing ADCresolution according to operation frequency and data from formationresistivity. Highly programmable analog systems can be used as theanalog core of software defined measurement systems and also be valuablein fast prototyping tool applications. Since subsurface induction toolsusually have various serial bus protocols for telemetry, a dependablesystem with flexibility to adapt easily to various protocols addsadditional value to the system.

Referring again to FIG. 1, generator 10 generates both sine wave andsquare wave signals. Generator 10 also provides reference signals forPSD (Phase Sensitive Detector) 20 operation, both in-phase andquadrature. Parallel tuning bank capacitors 12, permitting thereconfiguration of the hardware needed by software to run at differentfrequencies, output the desired frequencies at selected amplitudes andrepetition rates for energizing differentially driven single transmittercoil 13. Intelligent edge-rate control techniques are integrated intothe transmitter driver design to minimize the effect of parasiticcapacitance and resistance, manage EMI generation, minimize distortion,and still maintain high efficiency and optimal signal-to-noise ratio(SNR). This results in an electronics topology that is less sensitive tonoise. To monitor current stability, a voltage is applied acrossinternal reference 15 to account for electronic drifts with temperature.

Signals from the formation are sensed by a mutually balanced, fullydifferential sensor-receiver coil system 16 and are amplified by anultra-low noise preamplifier, which is coupled to selectable cut-offfrequency bandpass filter 18, which is synchronized with the frequencyof operation. The signals are then amplified again by amplifier 19 andpassed to phase-sensitive detector 20 to lock-in to the frequency ofinterest using the square wave reference signals coming from generator10. PSD 20 separates the auto induced signals X from the signals sensedfrom the formation R. Then, low pass filter 21 at the PSD outputgenerates the DC voltage of both signals X and R. High sampling rate24-bit ADC 22 with parallel channels then converts the signals todigital data. ADC 22 is controlled by high-speed microcontroller 23 andsends the data to master control 24 through high-speed communicationdata bus 25.

Referring to FIG. 4, the Thin Bed Induction (TBI) tool disclosed hereinis preferably composed of N pairs of receivers 101-105 (when there arefive pairs of receivers) and one extra channel (from coil 15, FIG. 1)for internal calibration purposes (to sense the transmitter current anddetect electronics drift). Additional receivers 106 may be added. Allreceiver sensors are placed in pairs, as discussed above. (Only one coilis shown in the figure, for clarity). The tool may perform multiple ADCconversions in parallel, independent of the number of receiver coils orsensors on the bus. The tool is capable of driving from the master thesignals from two to N transmitters in one sample cycle—in real time. Ifmore than one transmitter is used, it may be in a separate tool. Thisfeature gives the logging tool disclosed herein a flexible architecturethat can handle dense, complex information in short frame times. Thiscapability increases precision, resolution, and data correlation inthin-bed logging.

Measured signals are sent to master control 24 through data bus 25 (FIG.4). Master control 24 drives the logging tool, saves the data fromreceivers 102-105, synchronizes information between boards andreceivers, receives commands from up-hole through communication modules109 and sends data to communication modules 109.

Additional sensors 106 and actuators 107 may be included in the system.The sensors may be a mud sensor or accelerometer, for example. Theactuator may operate a motor, for example. The system may include USBprogramming testing module 108 and extra memory slots 110. It isreconfigurable and flexible enough to drive multiple receivers, multipletransmitters, and multiple frequency induction systems. The flexiblearchitecture of the system may use several communication protocols andinternal buses, such as I2C, RS485, CAN, USB, and TCP/IP.

The disclosed system may be driven and accessed through the I2C data busby a telemetry system such as disclosed in U.S. patent application Ser.No. 13/267,313, filed Oct. 6, 2011, or through other commercialtelemetry systems. The system preferably sends commands and receives rawdata from all receivers every 50 mS in logging mode and every 500 mS incalibration mode, for all interpretation and calculation algorithms usedup-hole in a surface logging unit.

FIG. 5 shows a master flow chart and a transmitter flow chart forsignals used in operation of the logging tool. A series of commandsbegins at 50 for sending different frequencies in succession totransmitters—for example, for 10 mS at each frequency, and receivingdata from receivers. These values may be varied by software. In theexample, the commands are: send frequency F1 for 10 mS, then sendfrequency F2 for 10 mS, then send frequency F3 for 10 mS, then groundthe circuits for 10 mS, then send acquired data to the master controllerin 10 mS, then begin a new cycle. Thus, the cycle time is 50 mS. Thecurrent from the transmitter (internal reference) is read every 500 mS,or 10 cycles. Data from receivers and the internal reference signal(current from the transmitter) are received at 51 and stored in RAM 52.The stored data are then sent uphole in response to a command from themaster controller. Timers are set in response to commands from themaster controller at 53. At 54 a sequence is started to set operationalfrequencies by setting tuned capacitors, generate PWM (Pulse WidthModulation) for transmitters, generate square waves and read the ADC(Analog to Digital Converter) to measure current consumption from thepower supply.

Although the present invention has been described with respect tospecific details, it is not intended that such details should beregarded as limitations on the scope of the invention, except to theextent that they are included in the accompanying claims.

We claim:
 1. Apparatus for measuring electrical conductivity of a rock formation surrounding a wellbore, comprising: a housing adapted for operation of electronic apparatus in a wellbore; means for generating a linear frequency modulated multi-frequency sine wave at variable frequencies enveloped within pulse compressed time intervals and pulse widths; a differentially driven transmitter coil disposed at a selected location on the housing connected for receiving and transmitting the signals from the first electronic circuits; a plurality of mutually balanced fully differential receiver coils disposed at selected locations on the housing, each coil connected to second digitally-controlled electronic circuits for amplifying and filtering signals from the receiver coils; a phase-sensitive detector and low pass filter for receiving signals from the second electronic circuits and the first electronic circuits and producing in-phase and quadrature out-of-phase signals as an analog signal; an analog-to-digital converter to process the analog signal and produce a digital signal for transmission; and a programmable controller having a computer-readable medium programmed for controlling electronic apparatus in the housing.
 2. The apparatus of claim 1 further comprising a current sensor in the first electronic circuits for sensing current to the transmitter coil and sending a signal to an electronic circuit, the electronic circuit including a phase-sensitive detector and low pass filter for receiving signals from the current sensor and providing an internal reference signal.
 3. The apparatus of claim 1 wherein the phase accuracy of electronic circuits for generating sine wave and pulse signals are designed for stability over a range of wellbore temperature.
 4. The apparatus of claim 1 wherein the receiver coils are accessed, calibrated and synchronized in parallel by a real-time processor that collects data, drives the tool and communicates with a telemetry system.
 5. The apparatus of claim 1 wherein the steps of claim 4 are performed in a selected cycle time.
 6. The apparatus of claim 1 further comprising computer readable medium programmed with adaptive algorithms under firmware control to control stimulus and measurement components to optimize operating conditions of the system as determined by the algorithms.
 7. The apparatus of claim 1 further comprising sensors under control of the controller.
 8. The apparatus of claim 1 wherein the phase-sensitive detector is based on square wave reference signals generated in the first digitally-controlled electronic circuits for generating sine wave and pulse signals.
 9. The apparatus of claim 1 wherein the apparatus includes five pairs of receivers, each receiver having main and bucking coils in series.
 10. The apparatus of claim 1 wherein the sine wave signals are generated at three frequencies sequentially.
 11. The apparatus of claim 1 further comprising a second transmitter coil, which may be in a separate tool.
 12. The apparatus of claim 1 wherein an operation frequency is controlled by a variable capacitor, the capacitance of which may be selected such that the pulse frequency of operation is adjustable by firmware to obtain frequencies in the range from 1K to 32 KHz, and in time intervals of 10 msec or other desired time intervals.
 13. A method for measuring electrical conductivity of a rock formation surrounding a wellbore, comprising: providing a housing containing electronic apparatus, the electronic apparatus comprising a plurality of electronic circuits, the housing being adapted for use in a wellbore; operating an electronic circuit for generating and transmitting the signals from a differentially driven transmitter coil on the housing; operating an electronic circuit for receiving signals through a plurality of mutually balanced fully differential receiver coils on the housing; operating a phase-sensitive detector receiving signal from the generator producing pulse-compression signals and producing in-phase signals and quadrature signals; converting the in-phase and quadrature signals to DC and transmitting or storing the signals; and controlling the electronic circuits with a controller having software stored in a programmable medium.
 14. The method of claim 13 further comprising detecting current from the electronic circuit to the transmitter coil as an internal reference signal.
 15. The method of claim 13 wherein the controller is programmed to operate the electronic circuit to produce sine waves and pulses, the sine waves being produced in the sequence of a sine wave having a first frequency and amplitude, a sine wave having a second frequency and amplitude, and a sine wave having a third frequency and amplitude, all sine waves being within a pulse width, and further to send a signal to ground the circuits between the pulses.
 16. The method of claim 13 further comprising applying adaptive algorithms to the in-phase and quadrature signals to optimize operating conditions of the system.
 17. The method of claim 13 wherein the capacitance of the variable capacitor is selected to produce a frequency sweep or chirp mode of operation as frequency is adjusted. 